Ozone (O₃) is one of the best-documented air pollutants in East Asia and in many parts of the world. Tropospheric O₃ concentration has rapidly increasing in East Asia since the 1990s, and the global average tropospheric O3 concentration is expected to have increased by 50% by 2020 relative to the 1980s. Since the concentration of tropospheric O₃ > 40 nL L^-1 would cause visible leaf injury, plant damage and reduction in crop and forest productions, the effects of tropospheric O₃ on terrestrial ecosystems have aroused considerable attention the world over. Currently many of these studies have focused on effects of O₃ on plants, and most agreed that O₃ inhibits plant growth and accelerates plant senescence. Elevated O₃ has also been demonstrated to reduce photosynthetic rate and productivity of crops and forests, and to alter carbon metabolism and subsequently allocation of resources (e.g. Carbon (C)) underground. The increasing atmospheric O₃ concentration has a negative effect on the plant-soil system, thus further affecting the turnover of soil organic carbon pool. This is important as it is well known that soils are important C sinks within the biosphere. Soil organic carbon in biogeochemical cycling is divided into different fractions of active organic carbon according to the ease and time with which soil organic carbon becomes available in the soil, including easily oxidized organic carbon, protected slow organic carbon, non-protected slow organic carbon, and passive carbon. However, not so much is known about the effect of elevated O₃ on sequestration and stability of the different fractions of soil organic carbon. Thus it is important to better understand C cycles in the context of predicted increases in atmospheric O₃. The paddy fields of the Yangtze River Delta region in Southeast China are one of most heavily O₃ -polluted regions of the country. In light of the larger amount of carbon deposition to paddy soils than to other agricultural soils, it is essential to have a understanding of responses of soil organic carbon and sequestrations of different fractions of active organic carbon under elevated O₃. Thus the main objective of this study was to determine whether an increase in atmospheric O₃ concentration would influence soil organic carbon and sequestration of each active carbon fraction. With the Chan-modified free-air O₃ concentration enrichment system and Walkley-Black method, effects of elevated atmospheric O₃ on different active soil organic carbon stocks in paddy soil were investigated. The paddy field under investigation had been under a rice-wheat rotation agroecosystem with elevated atmospheric O₃, 50% higher than the ambient O₃, for five years. Results showed that elevated atmospheric O₃ significantly decreased the contents of soil organic carbon in the 0 ~ 3 cm and 10 ~ 20 cm soil layer, with a total decrease of about 18.4% in the topsoil (0 ~ 20 cm). Elevated atmospheric O₃ significantly decreased the contents of easily oxidized organic carbon in the 0 ~ 3 cm, 3 ~ 10 cm and 10 ~ 20 cm soil layers, but increased the content of protected slow organic carbon by 10.8%, while decreasing the content of non-protected slow organic carbon by 59.7% in the 0 ~ 3 cm soil layer, and the content of protected slow organic carbon by 59.6% in the 10 ~ 20 cm soil layer. The effects of elevated atmospheric O₃ on the proportions of different fractions of active organic carbon to total organic carbon related to fraction and soil depth. Elevated atmospheric O₃ significantly decreased the proportion of easily oxidized organic carbon to total organic carbon by 15.1% in the 3 ~ 10 cm soil layer, did not affect the contents and distributions of passive carbon in all soil layers, but caused the stock of labile organic carbon, accounting for 59.3% ~ 69.8%, in total soil organic carbon pool to decline, which is probably the direct cause leading to decrease in soil organic carbon under elevated atmospheric O₃. It is quite obvious that long-time exposure to elevated atmospheric O₃ would decrease the content of soil organic carbon and change the distribution patterns of different fractions of active organic carbon in soil carbon pool and their turnover.